In prior art, numerous different automatic exposure control (AEC) systems are known, which have been applied in connection with many different imaging solutions. In diagnostic X-ray imaging, exposure automation plays a very important role because its incorrect operation may lead to over- or underexposure and consequently to an unnecessary increase in the radiation dose received by the patient and to uncertain or even wrong diagnoses due to bad image quality. Incorrect exposures can of course be remedied via repeated imaging, but this further increases the radiation dose received by the patient and, on the other hand, it also involves extra work and expenses.
As the development of the film/intensifying screen combinations used in mammographic apparatus is making them increasingly sensitive and steep, automatic exposure solutions that have worked well to date may prove to be insufficient and they can not necessarily keep darkening of the film within acceptable limits, especially when operation of the automatic exposure system is being tested by varying the imaging parameters and object to the extent required by authority regulations.
The operation of automatic exposure systems currently in use is based on an empiric method in which each new film/intensifying screen combination is subjected to an enormous number of exposure tests using different imaging values of the imaging apparatus and varying the thickness, generally in range of 20–80 mm, of the acrylic plate typically used to represent the object to be imaged. Depending on the details of operation of the automatic exposure system used in each case, its various parameters are adjusted according to the results obtained until a sufficiently constant degree of film darkening is achieved in all circumstances. Each time when a new film/intensifying screen combination appears on the market or when more demanding tolerance requirements are set, such measurement series have to be carried out anew.
The commonly used calibration method as described above is thus based on radiographing a homogeneous plate, generally made of acrylic, simulating the object to be imaged. The periodic inspections carried out by the authorities to test the exposure automatics are based on the same method, which, it is true, is even necessary in respect of repeatability of the test. However, in an actual imaging situation, the object being imaged is not necessarily homogeneous. For example, the breast tissue imaged in mammography is by no means homogeneous, nor is the breast of standard size or shape in overall dimensions. As the object is typically also imaged from different projections in mammography, its shape and position in the imaging area may vary for this reason, too.
Already in early automatic exposure systems of mammographic apparatus, to allow for variation in the properties of the objects, the detector part of the automatic exposure system was movably arranged and the operator of the apparatus was expected to have the skill to select the optimal position for the detector for the measurement. Alternatively, the apparatus could have several detectors, of which the operator could choose the most suitable one. However, manual positioning of the detector involves certain problems and it may be incorrectly placed, e.g. partially outside the tissue to be imaged, with the result that radiation hitting the detector directly will stop the exposure too early. On the other hand, for example if the tissue being imaged contains a local dense part that happens to lie precisely in front of the detector, the exposure time may become too long. Therefore, especially in screening imaging where the imaging tempo is very rapid, incorrect detector positioning easily leads to errors regarding the degree of film darkening and thus may even create a need for repeated imaging as described above.
To eliminate the above-described problems regarding positioning of the detector, solutions using multiple detectors have been developed in which the signal produced by those detectors which have received quantity of radiation exceeding a given preset level is automatically left out of account. This limit level is so defined that, in order to reach it, the detector must obviously be located completely or partly outside the tissue being imaged. Solutions have also been developed in which a few detectors are used, of which the one is selected whose output signal appears to be the most suitable. While these solutions have significantly reduced exposure errors, they still involve the problem that their operation is only based on empirical knowledge that has proven to be good, and extending or adjusting such a function according to new and/or more demanding requirements may be very difficult and laborious, even impossible. In view of the latest proposals put forward by the authorities, in the future such requirements may include e.g. that the average film darkness should be determined with an accuracy of −+0.15 OD (Optical Density) (the figure is constant, but varies between 1.2 OD and 1.8 OD according to new research results and weightings varying from one country to another), but in the lightest areas of the film a certain minimum darkening should be guaranteed to ensure that a tumor, which is often located in exactly these dense areas of the tissue, can be diagnosed.